I think because when he is making Hats for the Red Queen and he get lost inside his head and Alice snaps him out of it he look like he really like him then
Originally the first script was a love story between the Mad Hatter and Alice, but it was changed. I've read the first script and really I would have preferred it more then the new one. Though Alice in Wonderland was still awesome and many people think of Alice and the Hatter as a couple.
In my opinion from The Hatters and Alice's chemistry I would say that they liked one another. Hatter was very protective of Alice and he always believed in her even though everyone else didn't. Alice felt the same way about Tarrant, they both had the same outlook on life and were both bonkers.
I read somewhere that they were thinking about making a number two, which I am all in for because number one just felt unfinished to me like a set up. They are just trying to see if they are going to be able to get everyone on board. Though this might be a rumor I know me and many of my friends hope its real.